Add a concurrency limit to Process.ProxyRequest (#123)

proxy/config.go

@@ -23,6 +23,9 @@ type ModelConfig struct {
 	UnloadAfter   int      `yaml:"ttl"`
 	Unlisted      bool     `yaml:"unlisted"`
 	UseModelName  string   `yaml:"useModelName"`
+
+	// Limit concurrency of HTTP requests to process
+	ConcurrencyLimit int `yaml:"concurrencyLimit"`
 }
 
 func (m *ModelConfig) SanitizedCommand() ([]string, error) {

proxy/process.go

@@ -57,10 +57,19 @@ type Process struct {
 	// for managing shutdown state
 	shutdownCtx    context.Context
 	shutdownCancel context.CancelFunc
+
+	// for managing concurrency limits
+	concurrencyLimitSemaphore chan struct{}
 }
 
 func NewProcess(ID string, healthCheckTimeout int, config ModelConfig, processLogger *LogMonitor, proxyLogger *LogMonitor) *Process {
 	ctx, cancel := context.WithCancel(context.Background())
+	concurrentLimit := 10
+	if config.ConcurrencyLimit > 0 {
+		concurrentLimit = config.ConcurrencyLimit
+	} else {
+		proxyLogger.Debugf("Concurrency limit for model %s not set, defaulting to 10", ID)
+	}
 	return &Process{
 		ID:                      ID,
 		config:                  config,
@@ -73,6 +82,9 @@ func NewProcess(ID string, healthCheckTimeout int, config ModelConfig, processLo
 		state:                   StateStopped,
 		shutdownCtx:             ctx,
 		shutdownCancel:          cancel,
+
+		// concurrency limit
+		concurrencyLimitSemaphore: make(chan struct{}, concurrentLimit),
 	}
 }
 
@@ -417,6 +429,14 @@ func (p *Process) ProxyRequest(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	select {
+	case p.concurrencyLimitSemaphore <- struct{}{}:
+		defer func() { <-p.concurrencyLimitSemaphore }()
+	default:
+		http.Error(w, "Too many requests", http.StatusTooManyRequests)
+		return
+	}
+
 	p.inFlightRequests.Add(1)
 	defer func() {
 		p.lastRequestHandled = time.Now()

proxy/process_test.go

@@ -340,3 +340,35 @@ func TestProcess_ExitInterruptsHealthCheck(t *testing.T) {
 	assert.Equal(t, "upstream command exited prematurely but successfully", err.Error())
 	assert.Equal(t, process.CurrentState(), StateFailed)
 }
+
+func TestProcess_ConcurrencyLimit(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping long concurrency limit test")
+	}
+
+	expectedMessage := "concurrency_limit_test"
+	config := getTestSimpleResponderConfig(expectedMessage)
+
+	// only allow 1 concurrent request at a time
+	config.ConcurrencyLimit = 1
+
+	process := NewProcess("ttl_test", 2, config, debugLogger, debugLogger)
+	assert.Equal(t, 1, cap(process.concurrencyLimitSemaphore))
+	defer process.Stop()
+
+	// launch a goroutine first to take up the semaphore
+	go func() {
+		req1 := httptest.NewRequest("GET", "/slow-respond?echo=12345&delay=75ms", nil)
+		w := httptest.NewRecorder()
+		process.ProxyRequest(w, req1)
+		assert.Equal(t, http.StatusOK, w.Code)
+	}()
+
+	// let the goroutine start
+	<-time.After(time.Millisecond * 25)
+
+	denied := httptest.NewRequest("GET", "/test", nil)
+	w := httptest.NewRecorder()
+	process.ProxyRequest(w, denied)
+	assert.Equal(t, http.StatusTooManyRequests, w.Code)
+}